In order to increase safety of vehicles, an auto-cruising control system, or a driver's assistance system is under study. Aiming at supplementing a driver's sensory perception of road conditions, these systems are often equipped with a radar system observing circumstances surrounding vehicles. As these automotive radar systems, various radar systems such as a pulse radar, a pulse-compression radar (a spread spectrum radar), an FMCW (frequency modulated continuous wave) radar, and two-frequency CW (continuous wave) radar have thus far been proposed.
As represented by meteorological radars and defense radars, up to now, radar systems have in many cases been comparatively high in cost. In automotive radars, however, low-cost and simplified systems are requested for their widespread use. When from this viewpoint, the previously described respective radar systems are considered, the pulse radar and the pulse compression radar each require high speed signal processing, so that a price of the radar system have no choice but to soar, whereas the FMCW radar and two-frequency CW radar are systems each acquiring a predetermined range resolution even in comparatively low-speed signal processing, and are promising for a major system in the automotive radars.
Continuous-wave radars such as the FMCW radar and the two-frequency CW radar are systems that acquire a predetermined distance resolution, by modulating continuous wave frequencies within a certain frequency bandwidth (sweep width), by emitting modulated continuous waves toward an object, and by acquiring beat signals between reflected received waves and modulated continuous waves. For this reason, these systems have a problem in that they are liable to interference by radio waves reflected from the roadsurface or those from other automotive radar systems. Among solutions to these problems, there is a method of allocating a radio frequency band differing on a per-radar system basis. A method in which a sweep width differing for each radar system is allocated in this way, is referred to as frequency hopping.
The operating principles of such radar systems indicate that in order to achieve higher range resolutions, there is a need for broader sweep width. It is known in the art, for example, that achieving a range resolution of one meter requires a bandwidth of 150 MHz. As a result, with n radar systems being present, in order for each radar system to achieve the distance resolution of one meter, the bandwidths of 150×n (MHz) are to be required.
Meanwhile, the radio-related laws allocate radio bands on a per-applications basis. Given that a frequency bandwidth allocated for the automotive radar is on the order of 1 GHz, the maximum n that satisfies the equation of 150×n (MHz)<1 GHz will be six. That is, it is turned out that the frequency bandwidth of 1 GHz can contain only some six radar systems.
Also at present, a need for enhancing measurement accuracy in the automotive radar system tends to be increased, and the sweep width to be requested on a per-radar system basis tends to become wider. In the meanwhile, it is unacceptable that only maximum of six vehicles are allowed to run under current road circumstances. With the information technologies developing, radio frequency applications continue to expand, thereby the frequency bandwidth to be allocated for the automotive radar system cannot be anticipated to become broader. That is, interference to occur between radar systems tends to be worsened, which makes it difficult for the problem to be overcome by only the frequency hopping. The automotive radar system would be difficult to become sufficiently widespread unless an alternative solution replacing the frequency hopping is found.
As a solution to such problems, after pulsing continuous waves and code-modulating the phase between the pulsed waves, a method of mixing on the basis of code pattern of the phase has been proposed (e.g., Non-patent Document 1).
Non-Patent Document 1
Akihiro Kajiwara, “Stepped-FM pulse radar for vehicular collision avoidance,” IEICE B-11. Vol. J-81-B-11, No. 3. pp. 234-239, May, 1998